Add docstrings, Remove d_b_theta_r, Use asarray
Add docstrings, Remove d_b_theta_r, Use asarray

d_b_theta_r is not needed as derivates can be called from splines
objects.
asarray insures that functions also work with list, tuple, int and float
inputs.
Finish implmenting skin core current profile.

jensv/fluxtubestability,jensv/fluxtubestability